Study: Different Dispersant Surfactants Help and Hinder Oil-Degrading Bacteria

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Oil mixed with dispersant. (Photo from NOAA flickr stream)

Scientists studied the interactions of the oil-degrading bacterium Alcanivorax borkumensis with oil across oil-water interfaces that had varying amounts of different surfactants.

They found that not all surfactants aid the bacteria’s natural oil degradation processes. The surfactant Tween 20 worked synergistically with the bacteria and increased the available oil interface, allowing for more bacterial growth and oil degradation. Other surfactants inhibited the bacteria’s adherence to oil, limiting its biodegradation capacity. They published their findings in Langmuir: Interaction of Alcanivorax borkumensis with a surfactant decorated oil–water interface.

Dispersants are used to break up oil into discrete droplets and increase the available surface area for bacteria adhesion, which is a vital process in biodegradation. A. borkumensis live on surface waters in the vicinity of oil rigs and natural oil seeps. Its population grows in response to oil influxes such as happened during the Deepwater Horizon spill. Scientists from Brown University and the University of Rhode Island conducted this study to help fill a research gap about dispersant effects on the bacteria community. The team recorded changes in this bacteria’s growth rate, lag time, and number density at the oil-water interface containing low to high levels of these surfactants: CTAB (cetylytrimethylammonium bromide), lecithin (L-α-phosphatidylcholine), SDS (sodium dodecyl sulfate), AOT (dioctyl sulfosuccinate sodium salt), and Tween 20. They used Corexit EC9500A at the “recommended” 1:20 volume ratio in the oil.

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The study authors depict the interaction of A. borkumensis with dispersant surfactants at the oil-water interface. (Image used by permission from Arijit Bose)

Researchers observed that low levels of negatively charged (anionic) surfactants repelled A. borkumensis from the oil-water interface. However, when low levels of the neutrally charged (nonionic) surfactant Tween 20 were used, the lag time for bacterial growth decreased by half and their growth rate nearly doubled compared to the control case with no surfactant. The growth rates of A. borkumensis in the presence of Corexit EC9500A was no different than the control. The researchers also found that stable emulsions of Tween 20 and oil were formed at higher concentrations of Tween 20, increasing the available area for bacteria attachment. Bacteria growth was initially slow at these higher concentrations but, after presumably producing biosurfactant of their own, the bacteria were able to degrade oil at a faster rate than they did at lower Tween 20 levels.

The team recommended further investigation into the use of different surfactants such as Tween 20 to treat future oil spills.

The study’s authors are Michelle Bookstaver, Arijit Bose, and Anubhav Tripathi.

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This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the University of Rhode Island, Brown University and University of Florida for their project Multifunctional Colloidal Particles as Dispersants for Maximizing Biodegradation of Crude Oil.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.